More Stuff Done Wrong

I see this article on how to build boards is making the rounds on the internets. http://smalltrimarans.com/blog/?p=12289  People, don’t do a foil that way. The writer either has never pushed his boats or does not do math, or both.

kohlrudder-daggerboard-construction-2-300x230 Makes a nice sculpture but not for real boards.

 Stressform, which he uses to to form the boards, is a fine way to make many things including masts and hulls.  But not boards.  As you all know, a board is a heavily loaded beam.   With a huge point load usually about half way up it.  Typically deflection calculations govern on board design.  The board in the above link has thin plywood skins.  The skin of any board can do the majority of the beam work as it is farthest from the centroid.  But plywood has only about 1/8th the modulus of carbon fiber.  So it will be about 8 times more bendy.  Carbon fiber will have between 10 to 20 times the bending strength also.  Carbon fiber should be used on all boards with an aspect ratio of 3 or more, which are most of mine.

In this how-to, there is a lumber spar down the center.  And he adds carbon to it.  But it is at the centroid of the foil.  That is where material contributes the least to structure of any place on the foil.  So they put the carbon fiber where it is least useful.  Stuff done wrong.  The spar is supposed to also provide shear transfer between the two sides.  It does, but only for about 10 times the skin  thickness either side of the spar.  So most the plywood is also wasted as a beam in this scheme.  This why I have core everywhere in a board.  It transfers all the loads from one side to the other.

Finally, remember that point load  halfway up the board?  It can be several thousand pounds load on one point.  Compression strength perpendicular to grain of the plywood will be between 100 psi to 300 psi, depending.  Carbon fiber will be thousands of psi.  The board failures that I have seen have been compression side buckling.  This looks like buckling waiting to happen.

I have a new 14 page chapter on how to build foils in both the new CM Construction Manual and in the Composite Construction Manual.  I will put those online next week and make available here.  Just trying to do an intervention everybody, before its too late.  I already am too late on one guy.  He bought Kohler plans to an imaginary cat that was a textbook example of Things Done Wrong.  More on that soon.

Glazing Sticky Tape Update

After seeing several instances of Norton double sided sticky glazing tape on my cat builds, I had assumed that it was as good as the 3M VHB products.  Maybe even better as it had more thickness.

I have just been informed by Norton that their double sided adhesive tape is not actually adhesive and should not be relied upon.  They recommend using the 3M product.  I have a call in to the 3M rep to see if there is anything else I should know, over and above my posting here from last Spring.  The lunar lander will need a lot of this product.

How to demo: https://www.youtube.com/watch?v=9Fo-DxupzeQ

vhb-tape

 

 

Scarey

Was 36 years ago this week that I launched my first trimaran. Was my home for a couple of years.

I heard it held the Point Hudson Race record time for several years. I remember the race officials asking if we could prove we rounded the mark. We could. It was still daylight when we crossed finish line back in Seattle.

SMOHOLLAewing